The present invention relates to optical recording media and in particular, to a magneto-optical recording medium having a recording layer formed on a transparent resin substrate through which a light beam is introduced to the recording layer.
Recently, considerable effort has been given to the development of optical recording media. Such recording media offer an advantage over conventional magnetic tapes or disks in that they can store data information at high recording density to provide very large storage capabilities.
A disk-shaped optical recording disk of the type known as an optical disk or diskette can include a metallic recording layer which is formed on a transparent substrate with a pre-groove. The substrate requires a minimum birefringence to reduce the noise level of a light signal, as well as its transparency for light beam transmission. In order to meet such requirements and allow easy formation of the pre-groove in the substrate, a transparent resin material (e.g., polymethyl methacrylate, polycarbonate, epoxy, and the like) is preferred as the substrate material.
The above transparent resin material, however, has poor adhesive properties with a metal or metallic compound constituting a recording layer. Therefore, it is difficult to form a metallic recording layer on the transparent resin substrate. This is a serious obstacle to practical application of an optical disk. In particular, in a magneto-optical disk as one of the most promising optical recording media, this problem is more serious. This is since a magneto-optical disk adopts a rare earth-transision metal amorphous ferrimagnetic film (to be referred to as an "RE-TM film" hereinafter) as a recording layer, and the RE-TM film has poor adhesive properties with respect to the resin substrate. This problem is most urgent in the development of magneto-optical disks.
In order to solve the above problem, according to Japanese Patent Disclosure (KOKAI) No. 60-79543, an adhesive layer consisting of a polymer layer is formed between the recording layer and the transparent resin substrate to adhere them. The adhesive layer is deposited on the substrate with a pre-groove by a spin-coat method using a wet process (formation of the pre-groove after that of the adhesive layer is not practical in the manufacture of the optical disk).
However, if the wet process is adopted, it becomes very difficult to uniformly deposit the adhesive layer on the entire surface of the substrate having the pregroove. The thickness of the adhesive layer varies widely on the wall portions of the pre-groove, and becomes nonuniform on the bottom portion thereof. As a result, the deposited adhesive layer cannot satisfactorily transform a sectional shape of the pre-groove. Therefore, the pre-groove defined on the adhesive layer is deformed, thus degrading fundamental data read/write control characteristics of the optical disk (e.g., tracking, focusing, and random-access). In the proposed optical disk with the pre-groove, if the adhesive properties between the recording layer and the substrate are to be improved, this may degrade fundamental characteristics of the disk. Thus, the optical disk proposed in the above disclosure is not the solution to the above problem.